This invention relates to a direct-coupling control device for a torque converter in a vehicular automatic transmission, and more particularly, in the case where the transmission includes a shift position permitting a transmission gear ratio of medium or high speed stage to be held, the invention relates to a configuration adapted to control the direct-coupling or locked-up state of the torque converter in such shift position.
In a vehicular automatic transmission equipped with a fluid-type torque converter, the torque converter is usually direct-coupled or locked-up to eliminate fluid slippage loss of the torque converter. Such lock-up can cause the torque converter to lose one of its functions, i.e., a torque amplifying function. Therefore, the power efficiency of the torque converter is liable to deteriorate. In a shift position where a gear shift can be effected automatically, no problem will be caused because the gear is shifted to a lower speed stage in accordance with the amount of movement of the accelerator pedal. Meanwhile, in a shift position where a gear shift is not effected automatically, a problem of deterioration in the power efficiency will be caused. It is also known to release the lock-up in accordance with the certain amount of movement of an accelerator pedal even in the latter shift position where a gear shift is not effected. This is an effective means. However, if such a means is applied to all of the vehicle speeds, the fuel consumption is increased in a range greater than the above predetermined amount of movement of the accelerator pedal. Thus, it is required that the amount of movement is controlled to be variable in response to the vehicle speed instead to being held constant. But, to realize that control, a control device would be complicated in its configuration. The necessary installation space is enlarged.
The final object of lock-up technique is to realize the full-time lock-up (i.e., to permit the lock-up at all times during driving of the vehicle), so that, in a shift position where a gear shift is made automatically, the lock-up is preferably effected all over the range even in transmission gear ratios of medium and high speed stages. In a shift position where the transmission gear ratios of medium and high speed stages are held, however, the full-range lock-up is practically unfeasible because it will result in stalling of an engine i.e., engine failure. In practice, therefore, there must be provided a valve means for releasing the locked-up state at a vehicle speed below a certain value and a means for detecting the vehicle speed. It is required to accurately release the lock-up at a vehicle speed below the set value. Moreover, although a vehicle speed detecting means of oil hydraulic type can be used with the set vehicle speed being sufficiently high, but, in an attempt at lowering the set vehicle speed based on the foregoing full-time lock-up principle, the conventional vehicle speed detecting means of oil hydraulic type utilizing a centrifugal force is limited in its accuracy. This requires the use of an electrical vehicle speed detecting means and a solenoid valve operated in response to a signal from the means. Such an electronic control system is generally expensive. The use of the expensive electronic control system is not necessitated to prevent an engine failure in a drive position because the transmission gear ratio is shifted down automatically in that position. As a result, electronic control will be effected only in a shift position where the transmission gear ratio of medium or high speed stage is held, thereby resulting in an unbalance between the cost and the efficiency.